The present invention relates to a temperature detector employing a complementary metal oxide semiconductor ring oscillator made of CMOS integrated circuit as a temperature sensor.
Referring to FIG. 1 showing a conventional system of a temperature detector, the system comprises a CMOS ring oscillator for sensing the ambient temperature, a logic circuit 2 for converting the frequency of the ring oscillator 1 into a temperature data, a display device 3 and a power supply 4.
FIG. 2 shows a circuit of the ring oscillator 1 as a temperature sensor. The frequency of the ring oscillator 1 comprising a plurality of inverters may be expressed by the following formula; EQU f=1/2NCR
where N is the number of the inverter, C is capacitance and R is resistance of each stage. If the inverter is made of a CMOS, the capacitance C is generally represented by capacitors formed by a substrate, gate oxide and aluminum wires on the gate oxide. The capacitance has very little temperature characteristics. Resistance R is represented by on-state resistance of the MOS transistor and resistance by diffusion. The resistance has a positive temperature coefficient. Accordingly, the ring oscillator frequency f has a negative temperature coefficient.
Referring to FIG. 5 showing frequency-temperature characteristics of the ring oscillator, a line 5a is a line showing a typical value. When supply voltage V.sub.DD -V.sub.SS is 1.5 V,, the temperature frequency coefficient is about -0.5%/.degree.C. In other words, as seen from FIG. 5, the ratio of change of frequency (.DELTA.f) determined by the variation in output voltage to a specified frequency (f) divided by a temperature in degrees Celcius, yields a temperature-frequency coefficient of -0.5 percent per degree Celcius. However, such a conventional system has the following two disadvantages.
First, the ring oscillator frequency f varies with variation of supply voltage. More particulary, on-state resistance of the MOS transistor which affects the resistance R varies with the supply voltage. Therefore, voltage-frequency characteristics of the system are bad.
FIG. 7 shows frequency-voltage characteristics of the ring oscillator. A line 7a is a characteristic of the conventional ring oscillator, which is greatly influenced by decrease of voltage of a battery as voltage supply. Thus, if the ring oscillator is employed for a temperature detector, the variation of the supply voltage causes error in temperature detection, which means inaccuracy of the detector.
Secondly, the temperature-frequency coefficient of the ring oscillator is small. If the temperature coefficient of a ring oscillator frequency f is 0.5%/.degree.C. (for example, when f is 1 KHz), variation of frequency per temperature is 5 Hz/.degree.C. Therefore, the variation rate is too small to improve the accuracy of the detector. However, the frequency f of the ring oscillator cannot be increased in dependence on the necessity of low power consumption of CMOS transistors.